<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>必应遇到的雪花背景</title>
  <style>
    html, body, a, div, span, table, tr, td, strong, ul, ol, li, h1, h2, h3, p, input {
      font-weight: inherit;
      font-size: inherit;
      list-style: none;
      border-spacing: 0;
      border: 0;
      border-collapse: collapse;
      text-decoration: none;
      padding: 0;
      margin: 0;
    }
    html, body {
      font-family: "Segoe UI",Segoe,Tahoma,Arial,Verdana,sans-serif;
      background-color: #3D69A4;
      overflow: hidden;
      padding: 0;
      margin: 0;
      width: 100%;
      height: 100%;
    }
    .snow .snow_slice {
      position: absolute;
      left: 0;
      top: 0;
      width: 100%;
      height: 600px;
      pointer-events: none;
      transform-origin: 0% 0%;
      animation-timing-function: linear;
      animation-iteration-count: infinite;
      overflow: hidden;
      z-index: 2;
    }
    .snow .snow_1 {
      animation-name: snow_falling_fore;
      animation-delay: 0s;
      animation-duration: 50s;
      height: 100%;
      top: -100%;
      pointer-events: none;
    }
    .snow .snow_2 {
      animation-name: snow_falling_fore;
      animation-delay: 25s;
      animation-duration: 50s;
      height: 100%;
      top: -100%;
      pointer-events: none;
    }
    .snow .snow_3 {
      animation-name: snow_falling_mid;
      animation-delay: 0s;
      animation-duration: 70s;
      opacity: .5;
      top: -100%;
      height: 100%;
      pointer-events: none;
    }
    .snow .snow_4 {
      animation-name: snow_falling_mid;
      animation-delay: 35s;
      animation-duration: 70s;
      opacity: .5;
      top: -100%;
      height: 100%;
      pointer-events: none;
    }
    .snow .snow_drifter {
      width: 100%;
      height: 100%;
      background-repeat: repeat;
      background-image: url(./image/snow-static.png);
      pointer-events: none;
    }
    @keyframes snow_falling_fore {
      0% {
        top:-100%;
        opacity:0
      }
      1% {
        top:-100%;
        opacity:1
      }
      99.8% {
        top:100%;
        opacity:1
      }
      99.9% {
        opacity:0
      }
      100% {
        top:-100%;
        opacity:0
      }
    }
    @keyframes snow_falling_mid {
      0% {
        top:-100%;
        opacity:0
      }
      1% {
        top:-100%;
        opacity:1
      }
      99.8% {
        top:100%;
        opacity:1
      }
      99.9% {
        opacity:0
      }
      100% {
        top:-100%;
        opacity:0
      }
    }
    /* 四边 */
    .frost {
      height: 100%;
      width: 100%;
      position: fixed;
      top: 0;
      left: 0;
      overflow: hidden;
      pointer-events: none;
      z-index: 2;
    }
    .frost .frame_sprite {
      position: absolute;
      height: 100px;
      background-repeat: repeat-x;
      background-size: contain;
      background-position: center bottom;
      background-image: url(./image/border-ice.png);
    }
    .frost .frame_left {
      top: -133px;
      left: 0;
      transform: rotate(90deg);
      transform-origin: 0 100px;
      pointer-events: none;
      width: 500%;
    }
    .frost .frame_right {
      top: 100%;
      left: calc(100% - 100px);
      transform: rotate(-90deg);
      transform-origin: 0 0;
      width: 500%;
      pointer-events: none;
    }
    .frost .frame_top {
      top: 0;
      width: 500%;
      transform: rotate(180deg);
      pointer-events: none;
    }
    .frost .frame_bottom {
      bottom: 0;
      left: 0;
      width: 500%;
      pointer-events: none;
    }
  </style>
</head>
<body>
  <div class="snow">
    <div class="snow_slice snow_1">
      <div class="snow_drifter drift_1"></div>
    </div>
    <div class="snow_slice snow_2">
      <div class="snow_drifter drift_2"></div>
    </div>
    <div class="snow_slice snow_3">
      <div class="snow_drifter drift_3"></div>
    </div>
    <div class="snow_slice snow_4">
      <div class="snow_drifter drift_4"></div>
    </div>
  </div>
  <div class="frost">
    <div class="frame_sprite frame_left"></div>
    <div class="frame_sprite frame_right"></div>
    <div class="frame_sprite frame_top"></div>
    <div class="frame_sprite frame_bottom"></div>
  </div>
</body>
</html>